I spent many months in pain management and I would not have been able to relax if there were pictures of syringes, etc. in the treatment room or the doctors office. I strongly agree that something in greens would be soothing from a patients point of view. As for the subject matter, some people find mountains relaxing, others like rivers or streams, I personally like tropical islands with palm trees and blue water and sandy beaches soothing. So something lovely in nature might be a good idea. Even though your gift is meant to be a surprise, asking him what he would like should be ok because just telling him you are making it for him will be a surprise in itself. He may enjoy it more if it has his personal touch in the design.